(Meth)acrylic acid crystal and process for producing aqueous (meth)acrylic acid

ABSTRACT

The present invention relates to processes for purifying a composition containing (meth)acrylic acid, at least one impurity and water, wherein the composition has a water content in the range of 0.55 to 90, based on the composition, to form a purified phase containing (meth)acrylic acid and at least one impurity, wherein, in the purified phase, the quantity of at least one impurity is less than 7% by weight, based on (meth)acrylic acid in the purified phase.

This application is a national stage application under 35 U.S.C. 371 ofinternational application no. PCT/EP03/02753 filed Mar. 17, 2003, whichis based on German Application no. DE 102 11 686.5, filed on Mar. 15,2002, and claims priority thereto.

BACKGROUND OF THE INVENTION

The present invention relates to a process for purifying a compositioncontaining (meth)acrylic acid, an apparatus for producing pure(meth)acrylic acid, an apparatus for polymerizing (meth)acrylic acid,(meth)acrylic acid and polymers obtainable by these processes, their useand substances containing them.

“(Meth)acrylic acid” is used in this text for compounds having thenomenclature names “methacrylic acid” and “acrylic acid”. Of the twocompounds, acrylic acid is preferred according to the present invention.

It is often desirable to work up (meth)acrylic acid to high purities ofat least 99.9% by weight for their use in polymers. In the hygieneindustry, for example, superabsorber polymers based on polyacrylates maycontain specific by-products only below the detection limit.

Crystallization is mentioned as an alternative to the production ofhigh-purity organic substances. The two particular processes, which areused industrially, are suspension crystallization and layercrystallization (Wintermantel et al, Chem. Ing. Tech. 1991, 63, 881-891;Steiner et al, Chem. Ing. Tech. 1985, 57, 91-102).

However, a crystallization step alone is often insufficient to removeby-products sufficiently well from the crystals as microinclusions ofmother liquors or the incorporation of impurities at crystal defects,etc. cannot be ruled out under finite crystal growth conditions. Theadhesion of mother liquor to the crystal can also impair the purity ofthe products.

For this reason, the crystals produced, particularly in the case of acrystal suspension, are frequently washed with washing fluids afterseparation from the mother liquor and/or the crystals are subjected,during layer or suspension crystallization, to an exudation processduring which impurities of any type may optionally be depleted. Aprocess of this type may be carried out continuously in what are knownas wash columns. The dissertation by Poschmann (ZurSuspensionskristallisation organischer Schmelzen und Nachbehandlung derKristalle durch Schwitzen und Waschen, Diss. Uni. Bremen, Shaker VerlagAachen 1996) provides an overview.

EP 0616998 discloses a process for producing at least 99.9% by weightacrylic acid starting from previously purified product having an acrylicacid content of 97.771% by weight. The effect of purification isachieved by the cooperation of dynamic and static layer crystallizationprocesses. What is known as falling film crystallization is adopted asthe final means of crystallization. An apparatus of this type may onlybe operated discontinuously and necessitates complex apparatus andlogistics and comparatively high-energy consumption owing to the manyprocess cycles required to obtain the necessary purities.

It is disclosed in WO 99/14181, to crystallize crude (meth)acrylic acidfor purification in a first step and to work it up in a second step,optionally using wash columns. The process disclosed therein startsdirectly with the condensation products of catalytic gaseous phaseoxidation for the production of (meth)acrylic acid. With this process,it is disclosed that the mother liquor produced after washing andseparating the crystals is recycled into the condensation stage. Withthis process, a product having a purity of 98.8816% by weight wasobtained from 90.972% by weight acrylic acid. However, this is notsufficient for some industrial applications. For example, the content ofinhibitors and aldehydes in the pure (meth)acrylic acid is a criticalvariable, which, if exceeded, may lead to drawbacks, for example in thesubsequent polymerization process.

Nienrood et al have disclosed how acrylic acid may be well purified bysuspension crystallization and subsequent treatment in a hydraulic washcolumn (so-called TNO process; Proc. Bremer International Workshop onIndustrial Crystallization, Bremen, 1994, Ed.: J. Ulrich, p. 4-11;Purification Potential of Suspension Growth Melt Crystallization, Proc.4th International Workshop on Crystal Growth of Organic Materials,Bremen, 1997, Ed.: J. Ulrich, Aachen Shaker Verlag, p. 139-145). Theacrylic acid used in these experiments was obtained from Aldrich and hada purity of 99.75% by weight. It could be purified to a purity of 99.97%by weight by this process. However, the use of lower-purity, acrylicacid has not been disclosed.

With former processes for producing (meth)acrylic acid, the(meth)acrylic acid obtained in a (meth)acrylic acid reactor andsubsequently dissolved in water in a quench absorber is usuallysubjected to a complex distillation process, which sometimesnecessitates the use of entrainers such as toluene so that high degreesof purity are then obtained by a crystallization process. Distillationprocesses for (meth)acrylic acid have the drawback that the(meth)acrylic acid is exposed to heat during the distillation process,through which its properties change, for example due to partial(pre)polymerization.

BRIEF SUMMARY OF THE PRESENT INVENTION

The object of the present invention is generally to overcome thedrawbacks of the prior art by providing appropriate technical teaching.

According to a further object of the present invention, a process forproducing high-purity (meth)acrylic acid from an impure crude(meth)acrylic acid stream from a process for producing (meth)acrylicacid is provided. The process should be harmless to the (meth)acrylicacid, industrially practicable and therefore superior to processes ofthe prior art from an economic and ecological point of view. In thisconnection, an excellent purification capacity and compliance withstringent ecological and economic requirements should be mentioned inthis connection.

A further object of the present invention is to provide an apparatus forproducing high-purity (meth)acrylic acid, which allows the purificationof impure (meth)acrylic acid to maximum purity with low energyconsumption and uninterrupted, environmentally sound operation.

A further object of the present invention is to provide a process and anapparatus wherein the risk of uncontrolled polymerization of(meth)acrylic acid during the production and, in particular, during thepurification of (meth)acrylic acid is reduced.

An object of the present invention is also to reduce the quantity ofstabilizers required to stabilize the (meth)acrylic acid betweenproduction and further processing of the (meth)acrylic acid.

A further object is to provide a further use of the apparatus forproducing the (meth)acrylic acid or one of its components in which therisk of undesirable, uncontrolled polymerization of the (meth)acrylicacid is slight.

A further object is to provide a process and an apparatus for producingaqueous solutions of (meth)acrylic acid, which is as pure as possible.This object stands against a background, in particular, where aqueous(meth)acrylic acids are often used in solution, suspension or emulsionpolymerization during the production of (meth)acrylic acid-containingpolymers. It is of particular interest here that the energy consumptionassociated with exchanges of solvent in the individual steps ofsynthesis from monomer synthesis to polymerization is kept as low aspossible.

An additional object according to the invention is to provide an aqueous(meth)acrylic acid phase, which has a comparably high waterconcentration at as high a purity of (meth)acrylic acid as possible.Aqueous acrylic acid phases of this type are in this respectadvantageous because they can be used directly in the aqueouspolymerization of absorbent polymers without a further dilution step. Inthis way, the until now common removal of water by distillation,followed by the purification of the acrylic acid and by a readdition ofwater for the polymerization of the water-absorbing polymer, isshortened by a step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a further process diagram according to the present invention:

FIG. 2 is a process diagram according to the present invention with twoprocess stages connected in series;

FIG. 3 is a process diagram according to the present invention with twonested process stages;

FIG. 4 is a process diagram according to the present invention with oneprocess stage;

FIG. 5 is a process diagram according to the present invention with twoprocess stages;

FIG. 6 shows a crystal habit of pure acrylic acid;

FIG. 7 shows a crystal habit according to the invention of acrylic acidfrom a 90/10 w/w mixture of acrylic acid and water;

FIG. 8 shows a crystal habit according to the invention of acrylic acidfrom a 80/20 w/w mixture of acrylic acid and water; and

FIG. 9 shows a crystal habit according to the invention of acrylic acidfrom a 70/30 w/w mixture of acrylic acid and water.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

These objects are achieved with a process for purifying a compositioncontaining (meth)acrylic acid, at least one impurity and water, whereinthe composition has a water content in the range of about 0.55 to about90, preferably about 7 to about 50 and particularly preferably about 10to about 25, or also of about 10 to about 85, preferably of about 15 toabout 80, particularly preferably about 25 to about 75% by weight,respectively based on the composition, to a purified phase containing(meth)acrylic acid and at least one impurity, wherein, in the purifiedphase, the quantity of at least one impurity is less than about 7% byweight, preferably less than about 5% by weight, more preferably lessthan about 3% by weight and particularly preferably less than about 1%by weight, based on the (meth)acrylic acid in the purified phase,comprising a stage of the process which includes the following processsteps:

-   a) (meth)acrylic acid is crystallized from the composition while    forming a suspension containing a mother liquor and (meth)acrylic    acid crystals;-   b) (meth)acrylic acid crystals are separated from the mother liquor;-   c) at least a portion of the separated (meth)acrylic acid crystals    is melted to form a melt; and-   d) a portion of the melt is recycled to step a) or step b),    preferably step b), and wherein the portion of melt which is not    recycled is in the form of a separated (meth)acrylic acid.

Provided that d) is not followed by further stages or steps of theprocess, the separated (meth)acrylic acid forms a component of thepurified phase.

According to an embodiment of the process according to the presentinvention, the composition, as a composition which is rich in water, hasa concentration of (meth)acrylic acid with up to about 10% by weight ofimpurities in the range of about 45 to about 80% by weight and water inthe range of about 20 to about 55% by weight, based on the respectivecomposition. The aqueous compositions may originate from a quenchabsorber. The aqueous compositions may also be a mother liquor depletedin (meth)acrylic acid or the filtrate from a crystallization, of whichthe (meth)acrylic acid is still to be purified.

In the case of the composition which is rich in water, the suspensionformed in step a) preferably contains water crystals in addition to(meth)acrylic acid crystals. It is also preferred, if the melt containsabout 10 to about 90% by weight, preferably about 15 to about 70% byweight and particularly preferably about 25 to about 55% by weight,based on the melt, of water in the case of the composition which is richin water.

According to a further embodiment of the process according to thepresent invention, the composition, as a composition which is depletedin water, has a concentration of (meth)acrylic acid with up to about 10%by weight of impurities in the range of more than about 80% by weightand water in the range of less than about 20 to about 0.55% by weight,respectively based on the composition. These compositions which aredepleted in water preferably originate from thermal depletion,preferably distillation, of (meth)acrylic acid or from a previouscrystallization step which preferably corresponds to the processaccording to the invention.

In a further embodiment of the process according to the presentinvention, it is preferred if the purified phase, as a phase which isdepleted in water, contains at least about 30% by weight, preferably atleast about 55% by weight and particularly preferably at least about 75%by weight of (meth)acrylic acid and less than about 5% by weight,preferably less than about 2% by weight, particularly preferably lessthan about 1% by weight and more particularly preferably less than about0.5% by weight of water, based on the respective purified phase.

In a further embodiment of the process according to the invention, it ispreferred if the purified phase, as a phase which is rich in water,contains at least about 20% by weight, preferably at least about 30% byweight and particularly preferably at least about 70% by weight of(meth)acrylic acid and in the range of about 5 to about 80% by weight,preferably in the range of about 10 to about 70% by weight, particularlypreferably in the range of about 15 to about 30% by weight of water,based on the respective purified phase.

The phase which is depleted in water may be supplied, for example, to asolvent-free polymerization process or a polymerization process in whichthe solvent is added only before or during the polymerization process.On the other hand, the phase which is rich in water may be addeddirectly to a solution, suspension or emulsion polymerization process inwhich water is used as a solvent.

(Meth)acrylic acid may be obtained in the desired purities byappropriate combination of the process according to the invention,optionally in a plurality of stages.

For example, the (meth)acrylic acid obtained from the crystallization ofan aqueous composition may be further purified as (meth)acrylic acidwhich is depleted in water. The mother liquor thus formed as an aqueouscomposition may be further processed to a phase which is depleted inwater or is aqueous by the process according to the present invention.

With the process according to the present invention, it is preferred tocrystallize (meth)acrylic acid in step a) at least in part, preferablyto at least about 5% by weight, particularly preferably at least about40% by weight and more preferably at least about 70% by weight of thecrystals, to form a crystal with a crystal structure having a surfacewith at least one recess located on the surface, the crystal structurehaving an orthorhombic Bravais crystal lattice with an Ibam space group,crystallographic data a=about 9.952 A, b=about 11.767 A and c=about6.206 A (cf. R. Boese, D. Blaser, I. Steller, R. Latz, A. Baumen ActaCrystallogr., Sect (Cr. Str. Comm.), 55, 9900006, 1999).

It is also preferred with the process according to the present inventionif, in step a), the mother liquor comprises at least about 95% by weightof (meth)acrylic acid and water, the water concentration of the motherliquor being in the range of about 10 and about 90% by weight, inparticular about 15 and about 70% by weight, preferably about 20 andabout 40% by weight.

It is particularly preferred if the crystals have a tubular recess intheir longitudinal direction, this recess preferably beingdumb-bell-shaped, the openings of the tubular recess being greater onthe end faces of the approximately square crystals than in the sectionalface of the crystal lying parallel to the end faces.

It is also preferred with the process according to the invention ifcrystallization takes places in a suspension and not by the formation oflayers in which a plurality of individual crystals are connected to oneanother to form a compact layer which is rigid in comparison with acrystal suspension. This is preferably achieved in that crystallizationdoes not take place on a surface with corresponding temperaturecontrol—for example the wall of a layer crystallizer—with formation of acrystal layer. Instead, the crystals formed in the suspension are freelymobile in the liquid phase, such as the mother liquor, containing them.

The above-described recess advantageously complicates the inclusion ofimpurities such as mother liquor inside the crystal. The impurities areable to flow out or be washed out through the open recesses.

Even a high water content in the (meth)acrylic acid does not lead to anotherwise conventional increase in inclusions but to cavities which areopen on both sides. If a water-containing suspension with such crystalsis subjected to liquid/separation, the adhering mother liquor canadvantageously flow away. This purification effect may be used, inparticular, to considerably simplify the production of (meth)acrylicacid, to improve the quality of the (meth)acrylic acid produced in thisway and thus to obtain pure, optionally aqueous (meth)acrylic acids.

Aqueous (meth)acrylic acids of which the water content is generally inthe range of about 10 and about 90, preferably about 11 and about 50,particularly preferably about 12 to about 40% by weight and moreparticularly preferably of about 25 to about 35% by weight, based on thetotal quantity of water and (meth)acrylic acid, are produced byquenching the (meth)acrylic acid obtained by catalytic gaseous phasereaction with water in a quench absorber. In the past, waterconcentrations of this type had to be reduced almost completely in orderto achieve sufficiently pure (meth)acrylic acid by crystallization. Thisnecessitated, in particular, at least one distillation step whichimpaired the quality of the (meth)acrylic acid owing to incipientpolymerization caused by exposure to heat.

In one embodiment, the volume of the recess is at least about 5% byvolume, in particular at least about 10% by volume, preferably about 20%by volume, particularly preferably about 50% by volume, of the volume ofthe crystal. These large volumes of the recesses ensure that onlycomparatively few impurities can appear inside the crystal.

In a further embodiment of the present invention, the crystal comprisesat least one inclusion, the sum of the volumes of inclusions being lessthan about 30% by volume, preferably less than about 15% by volume andparticularly preferably less than about 5% by volume and more preferablyless than about 1% by volume, of the volume of the crystal. Owing to thesmall volume of impure mother liquor in the inclusions, impurities whichare unable to flow away or be removed by a washing process, in the(meth)acrylic acid, based on the volume of the crystal, are limited.

In a further embodiment, the crystal has a length of between about 0.001and about 5 mm, in particular between about 0.05 and about 0.5 mm,preferably between about 0.1 and about 0.2 mm, on average. The length isdetermined by an image analysis system from about 500 crystals selectedat random from photographs obtained using a light microscope. An imageanalysis system consisting of a light microscope with attached CCDcamera and a PC evaluating unit is used for this purpose, whereby a PCprogramme from Soft Imaging System (SIS, V3.1) is employed.

The composition used in the process according to the invention, inparticular for crystallizing a crystal according to the invention, ismade up of at least about 60% by weight, preferably about 80% by weightand particularly preferably about 95% by weight of (meth)acrylic acidand water, the remainder being other substances, for example by-productsproduced during the synthesis of (meth)acrylic acid, wherein the waterconcentration is between about 0.5 and about 60% by weight, inparticular between about 5 and about 40% by weight, preferably betweenabout 15 and about 35% by weight.

This process surprisingly, but no less advantageously, leads tohigh-purity products which are also suitable for use in polymers, forexample in the hygiene industry.

The process preferably takes place continuously. It is preferable to usea wash column to separate the (meth)acrylic acid crystals from themother liquor. For this purpose, the wash column comprises a separatingregion in which the (meth)acrylic acid crystals are washed. Forsuccessful operation of a wash column, it is advantageous if thecrystals to be washed are hard enough and have a specific narrow sizedistribution in order to guarantee appropriate porosity and stability ofthe resultant packed or unpacked filter bed.

It is also preferred that the process according to the present inventionis characterised by a crystallization outlay (Wellinghof, Wintermantel,CIT 63 (1991) 9, p. 805, section 3.2.1) of about 1 to about 4.5,preferably less than about 1 to about 3.5 and particularly preferably ofabout 1 to about 2.5 and more preferably of about 1 to about 1.5.

The process according to the present invention may be carried out withimpure crude (meth)acrylic acid containing less than about 99.5% byweight of (meth)acrylic acid. The crude (meth)acrylic acid usedpreferably has a purity of about 50% by weight to about 95% by weight,preferably about 75% by weight to about 90% by weight of (meth)acrylicacid.

More preferably, the condensed mixture of a catalytic gaseous phaseoxidation for the production of (meth)acrylic acid may be introduceddirectly as a composition into the process according to the invention,preferably into step a).

Accordingly, (meth)acrylic acid, usually originating from acorresponding olefin, is generated in a reactor, then brought into anaqueous composition in a quench absorber, whereupon the (meth)acrylicacid may be distilled to a composition which is depleted in water in adistillation apparatus. The resultant crude (meth)acrylic acid stream issupplied to the purification apparatus. The process allows theproduction of very pure (meth)acrylic acid from comparatively impurecrude (meth)acrylic acid.

Crystallizing agents which allow the purification process according tothe invention to be carried out continuously should be used duringcrystallization. Suspension crystallization is preferably employed. Thismay be carried out advantageously in a stirred tank crystallizer,scraper crystallizer, cooling plate crystallizer, crystallizing worm,drum crystallizer, multitube crystallizer or the like. In particular,the variations of crystallization mentioned in WO 99/14181 may be usedfor this purpose. Crystallizers which may be operated continuously areagain particularly advantageous. Cooling plate crystallizers or scrapercoolers are preferred (Diss. Poschmann, p. 14). A scraper cooler ispreferably used for crystallization.

In principle, any wash column which allows continuous implementation ofthe purification according to the present invention may be used for theprocess according to the present invention. With a conventionalembodiment, the suspension in a hydraulic wash column is introduced inthe upper part of the column; the mother liquor is removed from thecolumn via a filter so that a close-packed crystal bed is formed. Thecrystal bed is permeated by the mother liquor in the direction of thebase of the column and pressed downward by the fluid resistance. On thebase of the column is a moving, preferably rotating, scraping device orscraper which produces a suspension again from the close-packed crystalbed and the washing melt introduced at the lower part of the washcolumn. This suspension is preferably pumped through a melter,preferably a heat exchanger, and melted. A portion of the melt may beused, for example, as washing melt; this is then recirculated into thecolumn and preferably washes out the crystal bed which is migrating inthe opposite direction, i.e. the crystallized (meth)acrylic acid iswashed in the counter current of the recycled (meth)acrylic acid. On theone hand, the washing melt brings about washing of the crystals, on theother hand, the melt crystallizes at least in part onto the crystals.The liberated crystallization enthalpy heats the crystal bed in thewashing region of the column. A purification effect similar to theexudation of the crystals is thus achieved. On the one hand,purification is therefore brought about by washing the surface of the(meth)acrylic acid crystals with molten, and therefore already purified,(meth)acrylic acid and, on the other hand, healing or exudation ofimpurities is achieved by crystallization of the molten purified(meth)acrylic acid on the existing (meth)acrylic acid crystals. Thisallows production of high-purity (meth)acrylic acid.

In a specific embodiment of the process according to the invention, thereturn flow resulting from the recycling of the molten (meth)acrylicacid from step c) into step a) or into step b) is greater than a feedflow of the composition which is continuously supplied to step a) fromthe exterior.

In particular, the return flow is at least twice and preferably at leastten times as great as the feed flow. The great return flow ensures thatthe exposure of the (meth)acrylic acid to heat on the melter is reduced.

For inoculation of the (meth)acrylic acid to be crystallized, it isadvantageous to supply the separated crystallized (meth)acrylic acidfrom step b) at least in part to step a). The recirculated (meth)acrylicacid crystals simplify crystal growth in step a) and therefore assistseparation of the (meth)acrylic acid from the mother liquor. This isadvantageous, particularly with an aqueous composition.

From the energy point of view, a single-stage purification process witha single stage of the process is particularly advantageous and thereforeparticularly preferred. However, a two-stage purification process may beappropriate under certain circumstances.

To increase the yield, it is expedient to recycle the mother liquorseparated in step b) at least in part to step a). (Meth)acrylic acidwhich has remained in the mother liquor may therefore be furthercrystallized with appropriate spatial temperature profiles, preferablyto the thermodynamic limit (for example eutectic).

In an advantageous group of embodiments, the process according to thepresent invention comprises at least two stages of the process, whicheach include steps a) to d), wherein at least one of the followingfeatures (α1) to (α4) is fulfilled:

-   (α1) separated, in particular crystalline and/or molten,    (meth)acrylic acid from a first stage of the process is supplied at    least in part to a second stage of the process;-   (α2) separated, in particular crystalline and/or molten,    (meth)acrylic acid from a second stage of the process is supplied at    least in part to a first stage of the process;-   (α3) mother liquor, in particular mother liquor separated in step    b), from a first stage of the process is supplied at least in part    to a second stage of the process;-   (α4) mother liquor, in particular mother liquor separated in step    b), from a second stage of the process is supplied at least in part    to a first stage of the process.

From this group of advantageous embodiments with at least one of thefeatures (α1) to (α4) of the invention, the embodiments in which atleast one of the following features (β1) and (β6) is fulfilled, arepreferred:

-   (β1) crystalline (meth)acrylic acid from the first stage of the    process is supplied to at least one of steps a) and b) of the second    stage of the process;-   (β2) molten (meth)acrylic acid from the first stage of the process    is supplied to at least one of steps a) and b) of the second stage    of the process;-   (β3) crystalline (meth)acrylic acid from the second stage of the    process is supplied to at least one of steps a), b) and c) of the    first stage of the process;-   (β4) molten (meth)acrylic acid from the second stage of the process    is supplied to at least one of steps a), b) and c) of the first    stage of the process;-   (β5) mother liquid separated in step b) from the first stage of the    process is supplied at least in part to step a) of the second stage    of the process;-   (β6) mother liquid separated in step b) from the second stage of the    process is supplied at least in part to step a) of the first stage    of the process.

If the second or further stage of the process brings about additionalpurification of the (meth)acrylic acid, i.e. (meth)acrylic acid alreadypurified in a first stage of the process is further purified in a second(further) stage of the process, the following embodiments from thisgroup of preferred embodiments of the present invention are particularlypreferred:

-   (1) for achieving high-purity (meth)acrylic acid, molten    (meth)acrylic acid from the first stage of the process is supplied    to step a) of the second stage of the process;-   (2) for achieving high-purity (meth)acrylic acid, crystalline    (meth)acrylic acid from the first stage of the process is supplied    to step a) of the second stage of the process;-   (3) to allow thorough washing of the (meth)acrylic acid crystals    from the first stage, molten (meth)acrylic acid from the second    stage of the process is supplied to step b) of the first stage of    the process; or-   (4) in order to provide particularly pure seed crystals, crystalline    (meth)acrylic acid from the second stage of the process is supplied    to step a) of the first stage of the process.

A combination of the respective embodiments (1) or (2) with (3) is mostparticularly preferred.

If the second stage of the process is used to increase the yield, thefollowing embodiments from the group of preferred embodiments of theinvention are particularly preferred:

-   (1) crystalline (meth)acrylic acid from the first stage of the    process is supplied to step a) of the second stage of the process as    seed crystals for crystallization;-   (2) to increase the yield, the mother liquor separated in step b) of    the first stage of the process is supplied at least in part to    step a) of the second stage of the process;-   (3) to increase the purity, molten (meth)acrylic acid from the    second step of the process is supplied to step a) of the first stage    of the process; or-   (4) to minimise energy consumption, crystalline (meth)acrylic acid    from the second step of the process is supplied to step a) of the    first stage of the process.

A combination of embodiments (2) and (4) is most particularly preferred.

Advantageously, at least two stages of the process are provided inseries. “In series” can refer both to the separated (meth)acrylic acid,i.e. to the crystalline or molten (meth)acrylic acid, and to theseparated mother liquor.

It is also advantageous to nest at least two stages of the process withone another. As a result, the number of melters which are required formelting and consume a large amount of energy during operation as heatexchangers or heaters, is smaller than the number of stages. Forexample, a simple nesting of two stages of the process can lead to asaving of one melter. Operation therefore becomes much more costefficient with an identical yield and identical purity.

The number of stages of the process is based on the purity and economicefficiency to be achieved by the process. The achievable purity of(meth)acrylic acid is limited by the thermodynamic limit (for exampleeutectic) for a crystallization capability of (meth)acrylic acid fromthe mother liquor.

A specific embodiment of the process according to the invention ischaracterised by the following features (γ1) and (γ2):

-   (γ1) crystallization of (meth)acrylic acid from an impure crude    (meth)acrylic acid stream from a process of producing (meth)acrylic    acid;-   (γ2) separation of the (meth)acrylic acid crystals from the mother    liquor using a wash column, wherein the mother liquor from step (γ2)    is recycled at least in part to step (γ1), the crude (meth)acrylic    acid stream having a purity of less than about 99.5% by weight of    (meth)acrylic acid.

The apparatus according to the present invention for producing(meth)acrylic acid comprises as components which are connected to oneanother in a fluid-conveying manner, a (meth)acrylic acid synthesisunit, which preferably includes a (meth)acrylic acid reactor and aquench absorber, a distillation apparatus and a purification apparatus,the purification apparatus including an apparatus unit comprisingfeatures (δ1) to (δ4):

-   (δ1) the apparatus unit comprises a crystallization region, a    separating region, a melter and at least three guides;-   (δ2) the crystallization region is connected to the separating    region by a first guide;-   (δ3) the separating region is connected to the melter by a second    guide;-   (δ4) the melter is connected to the crystallization region by a    third guide or to the separating region by a fourth guide;    wherein the purification apparatus comprises an inlet which guides a    composition containing (meth)acrylic acid, at least one impurity and    water, the composition having a water content in the range of about    0.55 to about 90, preferably of about 7 to about 50 and particularly    preferably of about 10 to about 25% by weight, based on the    composition.

According to the invention, the expression “in a fluid-conveying manner”means that gases or liquids, including suspensions, or mixtures thereofare guided through corresponding lines. In particular, pipes, pumps andthe like may be used for this purpose.

Owing to the low degree of distillation, the (meth)acrylic acid producedin this way is treated particularly carefully, whereby its quality isimproved. In addition, the pure aqueous (meth)acrylic acid obtained inthis way may be used in solution, emulsion or suspension polymerizationprocesses so that a detrimental exchange of solvent from identical ordifferent solvents may be avoided, in contrast to conventional reactioncontrol.

In an embodiment of the apparatus according to the invention, it ispreferred if the (meth)acrylic acid synthesis unit and the purificationapparatus are connected to one another without a distillation apparatus.

In a further embodiment of the apparatus according to the invention, itis preferred if the quench absorber and the purification apparatus areconnected to one another without a distillation apparatus.

An apparatus for producing acrylic acid which is preferred according tothe invention preferably has the following construction in the regioncomprising a (meth)acrylic acid reactor and a quench absorber during thesynthesis of acrylic acid: propylene and optionally further inert gasessuch as nitrogen or combustion gases such as carbon dioxide, CO₂, ornitrogen oxides are fed into a first reactor for initial catalyticoxidation via an educt inlet which opens into the first reactor. Thefirst reactor is connected by a further line to a second reactor intowhich the product of the first catalytic oxidation process from thefirst reactor is introduced for second catalytic oxidation. The acrylicacid-containing product of the second catalytic oxidation is fed via aline located between the second reactor and quench absorber to the lowerhalf of the quench absorber. In the quench absorber, the product ofsecond catalytic oxidation is brought into contact with water, the waterbeing introduced into the quench absorber above the supply of theproduct of the second catalytic oxidation.

On the one hand, a first phase which contains acrylic acid and water isdischarged from the quench absorber below the supply of the product ofsecond catalytic oxidation. This first phase may be recycled into thequench absorber again at least in part above the supply of the productof second catalytic oxidation. The first phase not recycled into thequench absorber is supplied to the distillation apparatus and subjected,for example, to azeotropic separation during which the acrylic acid isconcentrated and purified. A second phase which contains acrylic acidand water may be discharged from the quench absorber above the return ofthe first phase and below the introduction of water into the quenchabsorber.

This second phase, like the first phase, may be supplied to thedistillation apparatus and be subjected, for example, to azeotropicseparation during which the acrylic acid is concentrated and purified.The exhaust gases discharged from the quench absorber may be supplied toa catalytic combustion process. The combustion gases from catalyticcombustion may be introduced into the first reactor as inert gases. Thewater recovered during the concentration of acrylic acid may be recycledinto the quench absorber. Further details of the production of acrylicacid are disclosed in DE 197 40 252 A1.

According to the invention, acrylic acid may also be synthesised in asynthesis unit which converts propane directly into acrylic acid.Acrylic acid may additionally be synthesised in aqueous reaction media,preferably as homogeneous catalysis. During a reaction in aqueousreaction media, the acrylic acid would be produced in the form of thecomposition which is rich in water without the need to add water in aquench absorber.

An apparatus for the production of (meth)acrylic acid which is preferredaccording to the invention comprises the (meth)acrylic acid reactor anda quench absorber during the synthesis of (meth)acrylic acid bycatalytic gaseous phase reaction of C4 starting compounds with oxygen.(Meth)acrylic acid may be obtained particularly preferably by catalyticgaseous phase oxidation of isobutene, isobutane, tert.-butanol,iso-butyraldehyde, methacrolein or meth-tert.-butylether. Furtherdetails on the production of (meth)acrylic acid are disclosed in EP 0092 097 B1, EP 0 058 927 and EP 0 608 838.

The purification apparatus is capable of obtaining very pure(meth)acrylic acid with degrees of purity over about 99.5% by weightfrom a comparatively impure crude (meth)acrylic acid stream containingapproximately 85% by weight of (meth)acrylic acid. According to thepresent invention, it is possible efficiently to purify impure crude(meth)acrylic acid stream with about 50% by weight to about 95% byweight (meth)acrylic acid, preferably about 75% by weight to about 90%by weight (meth)acrylic acid. This effective purification allowspreliminary purification of the crude (meth)acrylic acid stream usingthe distillation apparatus to be reduced, so the heat exposure of the(meth)acrylic acid is reduced. The quality of the (meth)acrylic acid istherefore improved.

The apparatus unit comprises a separate purification apparatus for afurther increase in the purity of the (meth)acrylic acid. This separatepurification apparatus may be used for the further purification of theend product, in particular for the further purification of the(meth)acrylic acid leaving the melter.

To improve crystallization, the separating region is preferablyconnected to the crystallization region by a first return for separated(meth)acrylic acid.

To increase the yield, the separating region is advantageously connectedto the crystallization region by a second return for separated motherliquor.

In terms of energy, a single-stage purification apparatus with a singleapparatus unit is basically particularly advantageous and thereforeparticularly preferred. However, a two-stage purification apparatus maybe appropriate under certain circumstances.

To increase the efficacy of purification, in particular to increase thepurity and yield, the apparatus according to the invention contains atleast two apparatus units according to features (δ1) to (δ4), which areconnected by at least one connecting line, the connecting line being afeed line or a return line and at least one of the following features(ε1) to (ε4) being fulfilled:

-   (ε1) the separating region of a first apparatus unit is connected    via the connecting line to the crystallization region of a second    apparatus unit;-   (ε2) the melter of a first apparatus unit is connected via the    connecting line to the crystallization region of a second apparatus    unit;-   (ε3) the separating region of a second apparatus unit is connected    via the connecting line to the crystallization region of a first    apparatus unit;-   (ε4) the melter of a second apparatus unit is connected via the    connecting line to the crystallization region of a first apparatus    unit. The connecting lines may be a feed line as well as a return    line.

For example, it is expedient with the process according to the inventionto provide a feed line from the separating region of the first apparatusunit (first stage of the process) to the crystallization region of thesecond apparatus unit (second stage of the process) in order to increasethe yield. A return between the separating region of the second stageand the crystallization region of the first stage, on the other hand, isexpedient for the preparation of seed crystals.

Therefore, at least two apparatus units may advantageously be connectedin series to improve the purity and yield.

The number of melters required and therefore the energy required tooperate the apparatus may be reduced by nesting at least two apparatusunits.

With the process according to the present invention for purifyingacrylic acid, a temperature in the range of about −20 to about 20° C.,preferably from about −10 to about 13° C. at a pressure of from about 1to about 10 bar prevails in the separating region. It is preferred thetemperature and pressure prevailing in the lower region of theseparating region are lower than in the upper region of the separatingregion. Preferably, a temperature of from about −20 to < about 12° C. ata pressure of from about 1 to about 2 bar prevails in the lower regionof the separating region. A temperature of at least about 12° C. at apressure of from about 1 to about 10 bar, preferably from about 3 toabout 7 bar prevails in the upper region of the separating region.

With the process according to the invention for purifying acrylic acid,a temperature in the range of from about −20 to about 20° C., preferablyfrom about −12 to about 13° C. at a pressure of from about 0.5 to about10 bar, preferably from about 0.8 to about 2 bar prevails in thecrystallization region.

With the process according to the present invention for purifyingacrylic acid, a temperature in the range of from about 10 to about 50°C., preferably from about 11 to about 30° C. at a pressure of from about1 to about 10 bar, preferably from about 3 to about 7 bar prevails inthe melter.

With the process according to the present invention for purifyingmethacrylic acid, a temperature in the range of from about −5 to about30° C., preferably from about −3 to about 20° C. at a pressure of fromabout 1 to about 10 bar prevail in the separating region. It ispreferred that the temperature and pressure prevailing in the lowerregion of the separating region are lower than in the upper region ofthe separating region. Preferably, a temperature of from about −16 to<about 15° C. at a pressure of from about 1 to about 2 bar prevails inthe lower region of the separating region. A temperature of at leastfrom about 15° C. and a pressure of about 1 to about 10 bar, prevail inthe upper region of the separating region.

With the apparatus according to the invention for purifying methacrylicacid, a temperature in the range of from about −5 to about 30° C.,preferably from about −3 to about 20° C. at a pressure of from about 1to about 10 bar, preferably from about 1 to about 2 bar prevails in thecrystallization region.

With the apparatus according to the invention for purifying methacrylicacid, a temperature in the range of from about 10 to about 50° C.,preferably from about 11 to about 30° C. at a pressure of from about 1to about 10 bar, preferably from about 3 to about 7 bar prevails in themelter.

The temperature and pressure conditions prevailing in the guides allowreliable, uninterrupted transport of the (meth)acrylic acid and thesubstances possibly accompanying it in these guides.

The apparatus according to the invention enables a relatively impure(meth)acrylic acid to be used as the starting material, so thepreliminary expenditure for distillation of the (meth)acrylic acidoriginating from synthesis is reduced. Therefore, the exposure of the(meth)acrylic acid to heat, which may lead to undesirablepolymerization, is reduced.

The invention further relates to an apparatus for polymerising(meth)acrylic acid, comprising an apparatus for producing (meth)acrylicacid and a polymerization unit, wherein the purification apparatus ofthe apparatus for producing (meth)acrylic acid is connected to thepolymerization unit.

The invention also relates to a (meth)acrylic acid obtainable by theprocess of purification according to the present invention.

The present invention further relates to a process for producing apolymer containing (meth)acrylic acid, wherein a (meth)acrylic acidaccording to the present invention or a purified phase according to theinvention, obtainable by the process of purification according to theinvention, is polymerised. Polymerization preferably takes place assolution polymerization, reaction control in a recessed belt beingparticularly preferred. In the process, the aqueous phase is useddirectly or, alternatively, the phase which is depleted in water iscorrespondingly diluted. Polymerization generally takes place in amedium having a water content of about 20 to about 80% by volume, basedon the medium.

The invention also relates to a polymer which may be obtained by theprocess described in the foregoing paragraph.

The polymer is preferably an absorbent polymer with a maximum absorptionof about 0.9% by weight aqueous NaCl solution according to ERT 440.1-99in a range of about 10 to about 1,000, preferably about 15 to about 500and particularly preferably about 20 to about 300 ml/g. Further detailsabout absorbent polymers and their production may be found in “ModernSuperabsorbent Polymer Technology”, Fredrick L. Buchholz, Andrew T.Graham, Whiley-VCH, 1998.

The fibers, shaped articles, films, foams, superabsorbent polymers orhygiene articles according to the invention are based at least on orcontain at least the (meth)acrylic acid according to the invention orthe polymer according to the invention.

The (meth)acrylic acid according to the invention or the polymeraccording to the invention is used in or for the production of fibers,shaped articles, films, foams, superabsorbent polymers or hygienearticles.

The apparatus according to the invention or the purification apparatusaccording to the invention or the process according to the invention isused for producing acrylic acid having a purity of more than about 90%by weight, preferably more than about 95% by weight and particularlypreferably more than about 99.5% by weight, based on the respective(meth)acrylic acid with impurities.

In a specific embodiment of the process according to the invention forpurifying (meth)acrylic acid, the process comprises two process steps

-   γ1) crystallization of (meth)acrylic acid from an impure crude    (meth)acrylic acid stream from a process of producing (meth)acrylic    acid;-   γ2) separation of the (meth)acrylic acid crystals from the mother    liquor using a wash column, wherein the mother liquor from step (γ2)    is recycled at least in part to step (γ1), the crude (meth)acrylic    acid stream having a purity of less than about 99.5% by weight of    (meth)acrylic acid.

A mechanically operated wash column (Diss. Poschmann, p. 18) isparticularly preferred according to the present invention.

With a mechanical wash column—reference being made to EP 0 193 226 B andNL 100 76 87 A for example—a dense crystal bed is produced within thecolumn by means of a piston which is permeable to the melt. The pistonmay be located at the upper or lower end of the column; in the firstcase, the suspension is supplied in the upper region of the column andin the second case in the central or lower region. The piston ispermeable to the melt so, during compression, melt issues at the back ofthe piston, where it is removed. In a similar manner to a hydraulic washcolumn, the mechanical wash column also contains a scraping apparatus,for example a moving, preferably rotating, scraping member for scrapingcrystals from the crystal bed and transferring them into a suspensionwith the wash melt. The wash melt moves counter current to the crystalbed. Suspension is removed from the side of the wash column remote fromthe piston and, after the melting operation, a proportion of the meltmay be recycled as wash melt and the remainder removed from the circuitas extra-pure product.

Embodiments of suspension crystallization with subsequent washing of thecrystals in a hydraulic or mechanical wash column can be found in thebook “Melt Crystallization Technology” by G. F. Arkenbout, TechnomicPublishing Co. Inc., Lancaster-Basel (1995), p. 265-288 and the articleabout the preconcentration of waste water using the Niro freezeconcentration process in Chemie Ingenieur Technik (72) (10/2000),1231-1233.

Depending on the application, a washing fluid familiar to a personskilled in the art may be used as the washing fluid (for example waterin the case aqueous solutions). For washing the crystallized(meth)acrylic acid, as already mentioned, it is most particularlypreferred to use a proportion of the molten crystals thereof. Thisensures, on the one hand, that no further substance has to be introducedinto the system for the production of high-purity products and, on theother hand, the molten crystals are also used to force back the motherliquor front in the wash column and simultaneously have a purifyingeffect similar to exudation on the crystals. Products are not lost inthe process as the washing fluid crystallizes again on the crystals tobe washed and is therefore found in the product again (for examplebrochure from Niro Process Technology B.V., Crystallization and washcolumn separation set new standards in purity of chemical compounds).

It is further preferred in the process according to the presentinvention that the process occurs in a device according to theinvention.

In an embodiment of the process according to the invention, theseparated, in particular crystalline and/or molten, (meth)acrylic acidis purified in a separate purification process. For example, it ispossible to treat the mother liquor from step b) before the return tostep a), at least once by a further purification process. A personskilled in the art is sufficiently familiar with such processes.Preferably, the following processes in particular are employed:

1) Simple Distillation

-   -   Separation into low-boilers (acetic acid, water, etc.),        medium-boilers ((meth)acrylic acid) and high-boilers (MSA, PTA,        etc.). The purification of impure (meth)acrylic acid (in        particular of water and acetic acid) is carried out in the        majority of cases by azeotrope rectification. Entrainers such as        toluene or MIBK, for example, are used (EP 0 695 736 B1).        2) Extraction of Acrylic Acid    -   The (meth)acrylic acid may be obtained by extraction with        n-butanol. An aqueous phase in which the minor constituents are        dissolved remains. The extraction of (meth)acrylic acid from        impure solutions pertains to the prior art similarly to        distillation.    -   (Meth)acrylic acid may also be extracted, for example, from        aqueous solutions with liquid ion exchanges, mixtures of        tri-n-alkylamines and aliphatic alcohols or n-butanol (Vogel et        al.: Chem. Eng. Technol. 23 (2000) 1, pp. 70-74; Tamada et al.:        in Solvent Extraction 1990, Ed.: T. Sekine, Elsevier Science        Publishers B.V. pp. 1791-1796; JP 57 095 938; WO 98/40342;        Information brochure from SULZER Chemtech on fractional        extraction of (meth)acrylic acid with n-butanol).        3) Dewatering of (meth)acrylic Acid by Pervaporation    -   This process is disclosed inter alia in DE 4401405 A1.

Advantageously, the mother liquor from step b), prior to return to stepa), may be treated at least once by a process including steps a) and b)and may be recycled to the respectively purer branched partial stream instep a) of the original process or a prior process so that maximumpurity may be achieved with the minimum loss of yield.

FIG. 2 illustrates this matter. The mother liquor from crystalseparation in the wash column may also be treated in a subsequentcrystallization region. The resultant suspension may then be worked-upin turn in a wash column as usual. The mother liquor now formed may besimilarly treated in the following stages. There is a choice betweenrecycling the respective purer streams produced by this procedure intocrystallization of the first purification process or intocrystallization of a prior process. Maximum purity can thus be achievedwith a minimum of waste using relatively inexpensive apparatus.

The processes according to the invention may be carried out continuouslyand discontinuously, continuous operation being preferred as suchoperation is particularly cost effective.

The invention further relates to an apparatus for polymerising(meth)acrylic acid, comprising an apparatus according to the inventionfor producing (meth)acrylic acid and a polymerization unit, thepurification apparatus of the apparatus for producing (meth)acrylic acidbeing connected to the polymerization unit. This polymerization unit maybe a kneaded or stirred-tank reactor or a recessed belt in or on whichpolymerization takes place. Recessed belt polymerization is advantageousfor the phase which is rich in water. The phase, which is rich in water,has a water and (meth)acrylic acid concentration, which is particularlysuitable for the solution polymerization of polymers based on(meth)acrylic acid, preferably superabsorbent polymers and specialpolymers for leather and paper production and wastewater treatment.

The present invention further relates to a process for producing apolymer based on (meth)acrylic acid, whereby a (meth)acrylic acidaccording to the invention, or a purified phase, obtainable by theprocess according to the invention for the purification of (meth)acrylicacid, is polymerised. The polymers obtainable by this process arepreferably superabsorbers, special polymers for the wastewatertreatment, dispersion dye, cosmetics, textiles, leather treatment orpaper production industry.

The present invention also relates to fibers, shaped articles, films,foams, superabsorbers polymers, special polymers for waste watertreatment, dispersion dyes, cosmetics, textiles, leather treatment orpaper production or hygiene articles, at least based on or containing(meth)acrylic acid purified according to the invention or a polymeraccording to the invention.

The present invention finally relates to the use of the (meth)acrylicacid produced according to the invention or a polymer according to theinvention in or for the production of fibers, shaped articles, films,foams, superabsorbent polymers or hygiene articles, detergents orspecial polymers for the waste water treatment, dispersion dye,cosmetics, textiles, leather treatment or paper production industries.

“Superabsorbent polymers” are polymers which absorb water or aqueousfluids in a quantity which is a multiple of their inherent weight.Preferably, more than half of the superabsorbent polymers are based onacrylic acid as a monomer. Further details of superabsorbent polymers,their production and use in hygiene articles may be found in “ModernSuperabsorbent Polymer Technology”, Fredrick L. Buchholz, Andrew T.Graham, Wiley-VCH, 1998, the content of which will be referred to aspart of this disclosure. These superabsorbent polymers are preferablyincorporated into hygiene articles. Particularly preferred hygienearticles include diapers, incontinence article for adults and sanitarytowels.

Furthermore, the respective combinations of features emerging from thefeatures characterised by numerals hereinbefore represent individualembodiments of the present invention.

Further details and advantageous embodiments will be described in moredetail with reference to the following drawings which will illustratethe present invention by way of examples.

FIG. 1 is a process diagram in which the process (in one stage) will bedescribed in summary:

Stage 1

-   1. storage vessel-   2. feed line from 1 to 3-   3. suspension producer (e.g. cooling plate crystallizer, scraper    cooler)-   4. feed line from 3 to 5-   5. wash column, hydraulic or mechanical-   6. product circuit-   7. product circuit pump-   8. product circuit heat exchanger as melter-   9. product vessel-   10. product return to counter current washing wash column 5-   11. mother liquor return (optional)-   12. mother liquor vessel (reject)

The wash column 5, the suspension generator 3, the product circuit heatexchanger 8, the feed line 4, the product circuit 6 and the productreturn 10 correspond to the separating region, the crystallizationregion, the melter, the first guide, the second guide, the fourth or, ifcrystal formation takes place in the wash column, the third guide.

The melt which is to be separated and is in the liquid state, in otherwords controlled to a temperature just above equilibrium, is guided fromthe storage vessel 1 via the feed line 2 into the suspension generator3. Crystals having a suspension packing of about 5 to about 50%(preferably, about 20 to about 30%) are formed continuously in thesuspension generator 3 by cooling below the equilibrium temperature ofthe melt. The suspension is guided continuously through the feed line 4into the wash column 5 where it is separated into a liquid and a solidphase by moving or stationary filters (hydraulic or mechanical (seeabove)). The filtrate leaves the wash column 5 and is suppliedcontinuously to the mother liquor vessel 12. To increase the yield, atleast a portion of the filtrate may optionally also be recycled via themother liquor return 11 into the suspension generator 3.

The crystals in the wash column 5 are packed to a crystal bed and,depending on the type of wash column, are shaved off at the top orbottom by rotary blades. The shaved off product crystals are circulatedas a suspension in the product circuit 6 by the circulating pump 7 andmelt due to the introduction of the melt enthalpy with the melter orproduct heat exchanger 8.

A portion is recycled through the product return 10 into the wash columnas washing fluid for counter current washing. The remainder leaves theapparatus as product in the vessel 9.

If a minimum loss of product in the reject cannot be achieved with thissingle-stage procedure, the mother liquor from the first stage ofcrystallization may optionally be worked-up in one or more furtherstages of crystallization or with other purification agents.

FIG. 2 shows an example of implementation in a plurality ofcrystallization stages.

Stage 2/3 etc.

-   13. mother liquor vessel from first stage (=12)-   14. feed line from 13 to 15-   15. suspension generator stage 2 (e.g. cooling plate crystallizer,    scraper cooler)-   16. feed line from 15 to 17-   17. wash column, hydraulic or mechanical-   18. product circuit of second stage-   19. product circuit pump-   20. product circuit heat exchanger as melter-   22. product return to 17-   23. mother liquor return to second stage crystallizer (optional)-   24. mother liquor vessel of second stage (reject)-   25. mother liquor supply from second stage to 26-   26. suspension generator stage 3 (e.g. cooling plate crystallizer,    scraper cooler)-   27. supply of product from 26 to 28-   28. wash column, hydraulic or mechanical-   29. product circuit of third stage-   30. product circuit pump-   31. product circuit heat exchanger as a melter-   32. product return to 28-   33. mother liquor return to crystallizer of third stage (optional)-   34. feed line from 28 to 35-   35. mother liquor vessel (reject) of third stage-   36. optional supply to further purification stages

The suspension generator 15, the wash column 17, the product circuitheat exchanger 20, the feed line 16, the product circuit 18, productreturn 20, the mother liquor return 23 each correspond in the firstapparatus unit to the crystallization region, the separating region andthe melter, the first guide, the second guide, the third and fourthguide, of the second return.

The suspension generator 26, the wash column 28 and the product circuitheat exchanger 31, the product circuit 32, the product return 29, themother liquor return 33 each correspond, in the second apparatus unit,to the crystallization region, the separating region, the melter, thesecond guide, the third and fourth guide of the second return.

The mother liquor feed line 25 corresponds to a connecting line.

The mother liquor (12/13) from the first stage is partially or whollyguided into the suspension generator of the second stage 15. Crystalswith a suspension density of about 5 to about 50% (preferably about 20to about 30%) are formed continuously in the suspension generator 15 bycooling below the equilibrium temperature of the melt. The suspension isguided continuously via the feed line 16 into the wash column of thesecond stage 17 where it is separated into a liquid and a solid phase bymoving or stationary filters (hydraulic or mechanical, see above).

Once the crystals have been densified to a crystal bed, they are shavedwith rotary blades at the top or bottom, depending on the type of washcolumn 17. The shaved off product crystals are optionally (as in thefirst stage) circulated as a suspension in the product circuit 18 by thecirculating pump 19 and melt owing to the introduction of the meltenthalpy with the melter 20. A portion may be recycled 22 into the washcolumn as washing fluid for counter current washing. The remainder maybe recycled into the suspension generator of the first stage 3 as amolten product. The filtrate leaves the wash column 17 and is suppliedcontinuously to the mother liquor vessel 24. For a further increase inyield, it is optionally possible to recycle at least a portion of thefiltrate also via the mother liquor return 23 into the suspensiongenerator 15 and/or to continue working it up in a third stage.

For this purpose, the mother liquor is conveyed via 25 into a furthersuspension generator 26. The suspension obtained as mentionedhereinbefore is conveyed via 27 into the wash column 28 where it ispacked to a crystal bed and shaved off with rotary blades at the top orbottom, depending on the type of wash column 28. The shaved off productcrystals are optionally (as in the first stage) circulated as asuspension in the product circuit 29 by the circulating pump 30 and meltdue to the introduction of the melt enthalpy with the heat exchanger 31.

A portion may be recycled 32 into the wash column as washing fluid forcounter current washing. The remainder may be returned to the suspensiongenerator of the first stage 3 or the second stage 15 as molten product.

FIG. 3 shows a further preferred layout of the working up processaccording to the invention.

-   41. storage vessel-   42. suspension generator (e.g. cooling plate crystallizer, scraper    cooler)-   43. feed line from 42 to 44-   44. wash column, hydraulic or mechanical-   45. product circuit-   46. product circuit pump-   47. product circuit heat exchanger as heat exchanger-   48. product vessel-   49. product return to counter current washing in wash column 44-   50. feed line of the second stage equals mother liquor from stage 1    (from wash column 44)-   51. suspension generator stage 2 (e.g. cooling plate crystallizer,    scraper cooler)-   52. feed line from 51 to 53-   53. wash column, hydraulic or mechanical-   54. product from the second stage (suspension, mixed with the    original from 41 which is not guided in stage 1, but in stage 2)    which is guided in the scraper cooler of the first stage (42)-   55. product supply pump-   56. supply from the first stage (41) directly to the head of the    wash column 53 of the second stage-   57. mother liquor return to the crystallizer of the second stage    (51) (optional)-   58. mother liquor vessel of the second stage (reject)

The suspension generator 42, the wash column 44, the product circuitheat exchanger 47, the product circuit 45, product return 49, the feedline 43 each correspond, in the first apparatus unit, to thecrystallization region, the separating region, the separating region,the melter of the second guide, the third and fourth guide, the firstguide.

The suspension generator 51, the wash column 53, the feed line 52, themother liquor return 57 each correspond, in the second apparatus unit,to the crystallization region, the separating region, the first guide,the second return.

The feed lines 54, 50 correspond to connecting lines. The nesting shownhere by way of example advantageously eliminates the need for the secondapparatus unit to have a melter.

A particularly preferred variation of a two-stage or multiple-stagedesign guides the melt to be separated from the first stage 41 via thesupply line 56 and the pump 55 to the head of the wash column of thesecond stage 53 where the shaved off product crystals of the secondstage are guided into the suspension generator of the first stage 42 asa suspension 54. This variation has the advantage, in terms of energy,that melting in the second stage may be dispensed with and the crystalsnow in the first stage do not have to be frozen out again.

The counter current washing in the wash column of the second stage 53 isachieved using the melt from 41 via feed line 56 of the first stagewhich is very pure in comparison with the impurity concentration in thesecond stage and therefore allows washing which is similarly effectiveto washing with product.

Although a combination of a scraper cooler and a wash column is knownfor the production of high-purity organic substances, it could not beanticipated that this process is also very suitable for concentrating(meth)acrylic acid with starting purities of < about 99.5% by weight andwith a high water content, based on the quantity of acrylic acid. It wasfurther surprising that the content of impurities, such as furtherorganic carbon compounds, may be considerably reduced to values by theprocess according to the invention. Consequently, critical variablessuch as furfural, inhibitors, acetic acid or maleic acid may also bedepleted to uncritical values.

FIG. 4 shows a process diagram according to the invention with oneprocess stage and

FIG. 5 shows a process diagram according to the invention with twoprocess stages.

-   60. apparatus unit (first process stage)-   61. crystallization region-   62. separating region-   63. heat exchanger as melter-   64. second guide-   65. third guide-   66. first guide-   67. fourth guide-   68. separate purification apparatus-   69. first return-   70. second return-   71. connecting line-   72. connecting line-   73. connecting line-   74. (meth)acrylic acid reactor-   75. quench absorber-   76. distillation apparatus-   77. purification apparatus-   78. apparatus unit (second process stage)-   79. pump-   80. residual mother liquor outlet-   81. product outlet

FIG. 4 shows an apparatus according to the present invention for theproduction of (meth)acrylic acid with a (meth)acrylic acid reactor 74, aquench absorber 75, a distillation apparatus 76 and a purificationapparatus 77, the purification apparatus 77 containing a crystallizationregion 61 and a separating region 62. The crystallization region 61 andthe separating region 62 are close together, the crystallization region61 and the separating region 62 preferably being connected to oneanother directly, not by pipes. Preferably, crystal growth also takesplace in a single housing during washing of the crystals, in particularseparation of the crystals from the mother liquor.

The crystallization region 61 and the separating region 62 are connectedto one another by a first guide 66, which may be formed by the commonhousing. The separating region 62 is connected via a second guide 74 tothe melter 63 which melts the (meth)acrylic acid crystals separated fromthe mother liquor in the separation region. The molten (meth)acrylicacid is conveyed by a pump 79 either via a fourth guide 67 to theseparating region 62 or via a third guide 65 to the crystallizationregion 61. Advantageously, a separate purification apparatus 68 has thefunction of increasing the purity of the (meth)acrylic acid.

To promote crystallization in the crystallization region 61,(meth)acrylic acid crystals are supplied separately as seed crystals tothe crystallization region 61 by means of a first return 69 in theseparating region 62. The yield is increased in that the (meth)acrylicacid remaining in the mother liquor after incomplete separation in theseparating region 62 is recovered by supplying the mother liquor fromthe separating region 62 via a second return 70 to the crystallizationregion 61. The purified (meth)acrylic acid is removed from the circuitby means of a product outlet 81. Residual mother liquor which cannot beutilised is removed by means of a residual mother liquor outlet 80.

FIG. 5 shows a purification apparatus 77 with two apparatus units 60 and78. The purification apparatus 77 comprises two process stages. In orderto increase the achievable purity of the (meth)acrylic acid and theyield, the two apparatus units 60 are connected to one another byconnecting lines 71, 72 and 73.

With respect to the mother liquor, which is supplied from the firstapparatus unit 60 via a connecting line 73 to the second apparatus unit78, the two apparatus units 60 and 78 are connected in series.

To increase the purity of the (meth)acrylic acid which is obtained fromthe second process stage with the second apparatus unit 78, which puritymay be lower owing to the lower (meth)acrylic acid concentration in themother liquor supplied to the second apparatus unit 78, the(meth)acrylic acid obtained in the second process stage is supplied tothe crystallization region 61 of the first apparatus unit 60 via aconnecting line 72, for further purification. Improved purity of the(meth)acrylic acid obtained is ensured by this subsequent purification.

With respect to transport of the mother liquor, the second apparatusunit 78 follows the first apparatus unit 60. With respect to transportof the (meth)acrylic acid, the first apparatus unit 60 follows thesecond apparatus unit 78. The two process apparatus units 78 and 60therefore nest with one another.

FIG. 6 shows a crystal habit of pure acrylic acid at an equilibriumtemperature of about 12.7° C. and a crystallization temperature of about11.25° C. with a crystallization time of about 2 hours.

FIG. 7 shows a crystal habit of acrylic acid according to the presentinvention from a 90/10 weight in weight (w/w) mixture of acrylic acidand water with an equilibrium temperature of about 4.5° C., with astarting temperature of about 3.1° C. and a cooling rate of about 1 K/hwith a crystallization time after about 1.5 h. As the comparison ofFIGS. 6 and 7 shows, the crystals according to the present inventionhave a small number of inclusions but that the habit changes in such away that recesses from which mother liquor can advantageously flowappear on the respective upper side of the crystals. It can be seen thatnot all crystals are affected by this phenomenon.

FIG. 8 shows a crystal habit of acrylic acid according to the presentinvention consisting of a 80/20 w/w mixture of acrylic acid and waterwith an equilibrium temperature of about −2° C. and a startingtemperature of about −3.5° C. with a cooling rate of about 1 K/h and acrystallization time of about 1.5 h. It can be seen that all crystalsare affected and the size of the indentations increases.

FIG. 9 shows a crystal habit of acrylic acid according to the inventionconsisting of a 70/30 w/w mixture of acrylic acid and water with anequilibrium temperature of about −7.5° C. and a starting temperature ofabout −8.7° C. with a cooling rate of about 1 K/h and a crystallizationtime of about 1.5 hours. FIG. 5. It can be seen that general crystaldefects increase, the actual habit still being clearly discernible. Someof the recesses are so great that the crystals are internally hollow.The discharge of mother liquor is ensured and the washing out of furtherimpurities is permitted also in this case.

The present invention will now be described in more detail withreference to non-limiting examples.

EXAMPLE I

Crystallization of Acrylic Acid Which is Depleted in Water

In an apparatus corresponding to FIG. 1 with a scraper crystallizer forsuspension creation and a mechanical wash column with a piston arrangedat the bottom and an outlet for the purified melt arranged at the top,acrylic acid having the following composition (Table 1) was introducedinto the scraper cooler.

TABLE 1 Name Starting Composition Colour index — >700 Water % 13.17Acetic acid % 1.304 Furfural % 0.033 Benzaldehyde % 0.038 Propionic acid% 0.018 Acrolein % 0.012 Protoanemonine % 0.024 Acrylic acid % 85.140Hydroquinone % 0.028 Phenothiazine % 0.008 Dimeric acrylic acid % 0.144Maleic acid anhydride % 0.276 Remainder % —

Acrylic acid of the composition shown in Table 1 was used as thefeedstock (composition which is depleted in water).

The scraper cooler was cooled, whereby a crystal layer was formed in thescraper cooler at approximately −5° C. and shaved off by the rotaryshavers in the scraper cooler to form a suspension. The filtered motherliquor was invariable sluiced completely from the process. After about12 hours' operation, the liquid phase in the crystallizer (=filtratefrom the wash column) had the composition according to Table 2.

TABLE 2 Name Filtrate Colour index — >700 Water % 20.846 Acetic acid %1.855 Furfural % 0.045 Benzaldehyde % 0.078 Propionic acid % 0.011Acrolein % 0.042 Protoanemonine % 0.032 Acrylic acid % 75.991Hydroquinone % 0.122 Phenothiazine % 0.013 Dimeric acrylic acid % 0.467Maleic acid anhydride % 0.301 Remainder % 0.208

The crystal suspension removed from the crystallizer was packed in thewash column to form a compact crystal bed. On the upper side of thecrystal bed, the crystal bed was shaved off using a rotary shaver,pumped in the product circuit as a crystal suspension and melted by theheat exchanger. A portion was recycled into the crystal bed for countercurrent washing, in order to keep the washing front constant so thatneither product nor mother liquor strikes through the crystal bed. Theremainder was obtained continuously as the product. The mean values fromproduct analysis of the acrylic acid which was obtained about 1 to 3hours later as the sample from Table 2, in other words about 12 to 15hours after the beginning of the experiment are shown in Table 3 asexamples of the product qualities achieved.

TABLE 3 Name Product Colour index — 10 Water % 0.043 Acetic acid % 0.182Furfural % <0.0001 Benzaldehyde % <0.0001 Propionic acid % 0.006Acrolein % <0.0001 Protoanemonine % <0.0001 Acrylic acid % 99.764Hydroquinone % <0.001 Phenothiazine % <0.0001 Dimeric acrylic acid %0.006 Maleic acid anhydride % <0.005 Remainder % —

As shown in Table 3, the process according to the invention allows theproduction of high-purity acrylic acid.

The concentrations were determined per GC and the colour index by theDIN-ISO 6271 method. Water was determined to ASTM D 1364 and theinhibitors (MEHQ) to ASTM D 3125.

EXAMPLE II

Crystallization of Aqueous Acrylic Acid

760 g of a sample from a quench absorber sump of an acrylic acidapparatus having a composition according to Table 4 were introduced intoa stirred double shell vessel and cooled by a cryostat.

TABLE 4 Name Feed Water % 32.7 Hydroquinone % 0.059 Acetic acid % 3.617Furfural % 0.0235 Benzaldehyde % 0.0269 Propionic acid % 0.012 Acrolein% 0.0101 Protoanemonine % 0.0161 Acrylic acid % 62.586 D-acrylic acid %1.088 MSA % 0.042 Remainder % 0.269 Total minor constituents % 4.714

The sample introduced was cooled to 11° C. and then cooled further at acooling rate of 0.1 k/min. It was inoculated with 1 mg of ice and 1 mgof crystalline acrylic acid at −15° C. The solution then became cloudy.After a further 10 minutes, the suspension was separated into solid andliquid on a vacuum filter controlled to a temperature of 0° C. with a250 μm propyltex filter cloth. The resultant mother liquor (filtrate,380 g) has the composition shown in Table 5. This mother liquor is moreheavily depleted in acrylic acid than in water.

TABLE 5 Name Filtrate Water % 36.9 Hydroquinone % 0.072 Acetic acid %3.379 Furfural % 0.0251 Benzaldehyde % 0.0308 Propionic acid % 0.010Acrolein % 0.0113 Protoanemonine % 0.0170 Acrylic acid % 57.984D-acrylic acid % 1.047 MSA % 0.035 Remainder % 0.489 Total minorconstituents % 5.1162

Two different crystal habits (one clear white and one milky) withdifferent melting temperatures and melting behaviour were observed onthe vacuum filter. The crystals, having residual moisture of 214 g, onthe vacuum filter have the composition shown in Table 6.

TABLE 6 Crystals prior to Name washing (filter moist) Water % 23.4Hydroquinone % 0.039 Acetic acid % 2.364 Furfural % 0.0161 Benzaldehyde% 0.0183 Propionic acid % 0.009 Acrolein % 0.0092 Protoanemonine %0.0107 Acrylic acid % 72.926 D-acrylic acid % 0.799 MSA % unmeasurableRemainder % 0.409 Total minor constituents % 3.674

The filter-moist crystals according to Table 6 were washed with aquantity of 200 g of VE water controlled to a temperature of 0° C. Theresultant filter cake has the composition shown in Table 7.

TABLE 7 Crystals prior to Name washing (filter moist) Water % 38.8Hydroquinone % 0.010 Acetic acid % 1.005 Furfural % 0.0055 Benzaldehyde% 0.0067 Propionic acid % 0.004 Acrolein % 0.0024 Protoanemonine %0.0037 Acrylic acid % 59.707 D-acrylic acid % 0.393 MSA % 0.014Remainder % 0.048 Total minor constituents % 1.493

The quantities specified in the foregoing were determined by gaschromatography.

In this example, a significant depletion of the by-products accompanyingthe aqueous acrylic acid were achieved from synthesis of the acrylicacid, as demonstrated by comparison of the total minor constituents.

1. A process for purifying a composition comprising (meth)acrylic acid,at least one impurity and water, wherein the composition has a watercontent in the range of about 10 to about 85% by weight, based on thecomposition, to form a purified phase comprising (meth)acrylic acid andat least one impurity, wherein, in the purified phase, the quantity ofat least one impurity is less than about 7 by weight, based on(meth)acrylic acid in the purified phase, comprising a stage of theprocess which includes the following process steps: a) (meth)acrylicacid is crystallized from the composition while forming a suspensioncomprising a mother liquor and (meth)acrylic acid crystals; b)(meth)acrylic acid crystals are separated from the mother liquor; c) atleast a portion of the separated (meth)acrylic acid crystals is meltedto form a melt; and d) a portion of the melt is recycled to step a) orstep b) and wherein the portion of melt which is not recycled is in theform of a separated (meth)acrylic acid.
 2. The process according toclaim 1, wherein, in step a), (meth)acrylic acid crystallizes at leastin part to form a crystal with a crystal structure having a surface withat least one recess located on the surface, the crystal structure havingan orthorhombic Bravais crystal lattice with an Ibam space group,crystallographic data a=about 9.952 Å, b=about 11.767 Å and c=about6.206 Å.
 3. The process according to claim 1, wherein, in step a), themother liquor comprises at least about 60% by weight of (meth)acrylicacid and water, wherein the water concentration of the mother liquor isin the range of about 10 and about 90% by weight.
 4. The processaccording to claim 1, wherein the (meth)acrylic acid crystals are washedin the countercurrent of the recycled melt.
 5. The process according toclaim 1, wherein the melt is purified in a separate purificationprocess.
 6. The process according to claim 1, wherein the (meth)acrylicacid crystals from step b) are supplied at least in part to step a). 7.The process according to claim 1, wherein the mother liquor separated instep b) is recycled at least in part to step a).
 8. The processaccording to claim 1, wherein the process comprises at least two stages,which each comprise steps a) to d), wherein at least one of thefollowing features (α1)to (α4) is fulfilled: (α1) separate (meth)acrylicacid from a first stage of the process is supplied at least in part to asecond stage of the process; (α2) separate (meth)acrylic acid from asecond stage of the process is supplied at least in part to a firststage of the process; (α3) mother liquor from a first stage of theprocess is supplied at least in part to a second stage of the process;and (α4) mother liquor from a second stage of the process is supplied atleast in part to a first stage of the process.